Such separating devices are required in many oil and gas extraction wells. Mineral oil and natural gas are stored in naturally occurring underground reservoirs, the oil or gas being distributed in more or less porous and permeable mineral layers. The aim of every oil or gas drill hole is to reach the reservoir and exploit it in such a way that, as far as possible, only saleable products such as oil and gas are extracted, while undesired byproducts are minimized or even avoided completely. The undesired byproducts in oil and gas extraction include solid particles such as sands and other mineral particles that are entrained from the reservoir up to the borehole by the liquid or gas flow. Depending on the permeability of the geological layer and the formation pressure, the flow rates of the solids-laden liquid and gas flow can become very high, up to 15 m/sec, and in individual cases even higher.
Since the mineral sands are often abrasive, the influx of such solids into the production tubing and pump cause considerable undesired abrasive and erosive wear on all of the technical internals of the borehole. It is therefore endeavoured to free the production flow of undesired sands directly after it leaves the reservoir, that is to say while it is still in the borehole, by filter systems.
Problems of abrasion and erosion in the removal of solid particles from liquid and gas flows are not confined to the oil and gas industry, but may also occur in the extraction of water. Water may be extracted for the purpose of obtaining drinking water or else for the obtainment of geothermal energy. The porous, often loosely layered reservoirs of water have the tendency to introduce a considerable amount of abrasive particles into the material that is extracted. In these applications too, there is the need for abrasion- and erosion-resistant filters.
In oil and gas extraction, the separation of undesired particles is usually achieved today by using filters that are produced by spirally winding and welding steel forming wires onto a perforated basepipe. Such filters are referred to as “wire wrap filters”. Another commonly used type of construction for filters in oil and gas extraction is that of wrapping a perforated basepipe with steel screening meshes. These filters are referred to as “metal mesh screens”. Both methods provide filters with effective screen apertures of 75 μm to 350 μm. Depending on the type of construction and the planned intended use of both these types of filter, the filtering elements are additionally protected from mechanical damage during transport and introduction into the borehole by an externally fitted, coarse-mesh cage. The disadvantage of these types of filter is that, under the effect of the abrasive particles flowing at high speed, steel structures are subject to rapid abrasive wear, which quickly leads to destruction of the filigree screen structures. Such high-speed abrasive flows often occur in oil and/or gas extraction wells, which leads to considerable technical and financial maintenance expenditure involved in changing the filters. There are even extraction wells which, for reasons of these flows, cannot be controlled by the conventional filtering technique, and therefore cannot be commercially exploited. Conventional metallic filters are subject to abrasive and erosive wear, since steels, even if they are hardened, are softer than the particles in the extraction wells, which sometimes contain quartz.
There is therefore a great need to counter the abrasive flows of sand with abrasion-resistant screen structures.
DE 10 2008 057 894 A1, WO2011/009469 A1 and WO2011/120539 A1 propose filter structures in which the filter gaps, that is to say the functional openings of the filter, are created by stacking specially formed densely sintered annular discs of a brittle-hard material, preferably of a ceramic material. In this case, at least three spacers are arranged on the upper side of annular discs, distributed uniformly over the circular circumference of the discs, and the discs are stacked one on top of the other in such a way that the spacers respectively lie one over the other.
The spacers are in the form of spherical segments. However, forming the spacers as spherical segments has the disadvantage that the ceramic materials that have very good resistance to abrasion and erosion, such as densely sintered silicon carbide, are sensitive to point pressure loading and, when subjected to excessive stress as a result of the point pressure loading, fail due to rupturing. High point contact loads are referred to as Hertzian stress. In the volume of material underneath the point that is under compressive load, high tensile stresses occur as a result of the point pressure loading and can lead to rupturing of the ceramic rings.
In the normal operating state, the separating device only experiences insignificant differences in pressure between the inlet side and the outlet side of the filter. This is the case as long as the separating device is not plugged, i.e. clogged, and can be flowed through more or less freely. The differences in pressure or pressure losses in the separating device are low under normal operating conditions. If, however, the filter gaps are plugged, the differences in pressure can increase very sharply.
One reason for the plugging or clogging of the separating device may be undesired lodgement of mineral particles at the inlet opening of the filter, that is to say at the annular gaps on the outer circumference of the annular stack. Among the factors on which the risk of plugging depends are the particle size distribution of the mineral particles/liquid mixture and the flow rate at the location of the filter.
Another reason for the plugging or clogging of the separating device may be that the borehole is intentionally filled with highly viscous liquids laden with solids. Such a liquid is referred to as a “fluid loss control pill”.
Then, depending on the operating conditions in the borehole, the plugged or clogged filter may be exposed to very great differences in pressure, which are of the order of magnitude of 2500 psi (corresponding to 172 bar or 17.2 MPa) external pressure, that is to say exposure to pressure from the outside, and 1000 psi (corresponding to 69 bar or 6.9 MPa) internal pressure, that is to say exposure to pressure from the inside.
External pressure loading occurs for example when the filter becomes plugged by undesired lodgement of mineral particles at the inlet opening of the filter, internal pressure loading occurring for example when the plugged filter is cleaned by flushing.
The users of filters therefore have a justified interest in taking the pressure resistance of filters into account in the design, and measuring it by a standard method.
These circumstances gave rise to the development of the measuring standard ISO 17824, First Edition, 2009 Aug. 15, for determining the pressure resistance of such filters. The filter is in this case subjected to internal pressure (burst pressure test) or external pressure (collapse pressure test) by using a viscous liquid laden with solids in tWOtest setups. In these tests, the pressure is increased until, as a result of the effect of the pressure, the filter allows coarser particles than correspond to the filter width to pass through, which is evident from a drop in pressure in the filter or in the feed line of the measuring fluid. This event is also referred to by the technical term “loss of sand control”, LSC for short.
The structural design of the filters according to DE 10 2008 057 894 A1, WO2011/009469 A1 and WO2011/120539 A1 has the effect that, in the tests according to ISO 17824, when the pressure builds up local pressure breakthroughs occur in portions of individual filter gap openings. These pressure breakthroughs can be explained by the bridge-forming solid particles of the measuring fluid being forced through the filter gap as a result of a too high pressure, which in turn causes an increase in pressure in the filter gap. The bridges formed by the solid particles collapse under the loading of the pressure. The liquid pressure then prevailing temporarily in the filter gap causes great axial forces, which put an axial load on the annular disc segments lying on both sides of the breached filter gap and also great flexural stress, so that there is the risk of the rings rupturing.
When testing the filters proposed in DE 10 2008 057 894 A1, WO2011/009469 A1 and WO2011/120539 A1 for their internal and external pressure resistance (burst pressure test, collapse pressure test) according to ISO 17824 and also when using them in production, there may be pressure conditions that lead to very great axial forces in the ceramic annular stacks. Even in the case of comparatively low isostatic pressures, the axial forces may increase to such an extent that rupturing of the rings occurs due to the Hertzian stress caused by the point contact on the spherical segments.
Configuring the spacers in the form of spherical segments has further technical and commercial disadvantages. Since rings with spacers formed in such a way cannot be reworked cost-effectively after sintering, the planarity of the annular discs and the height of the spherical segments must comply exactly to the prescribed specification, since otherwise the rings cannot be used and have to be discarded. Even when keeping within the technically possible tolerances, ceramic components that are said to be “as sintered”, i.e. not reworked, have greater tolerances than those that have been reworked by hard machining. Consequently, close tolerances of the filter width cannot be cost-effectively achieved with the rings that have spacers in the form of spherical segments. The disadvantages also include that a specially adapted pressing tool has to be available for every filter width to be produced. At least the upper punches of the pressing tool must be adapted to the height of the spherical segments, and consequently to the intended filter width, which entails considerable commercial disadvantages.
A further disadvantage of the structural designs proposed in DE 10 2008 057 894 A1, WO2011/009469 A1 and WO2011/120539 A1 concerns the compression springs. These compression springs, configured as spiral springs, are intended to keep the pre-loading of the ceramic annular discs constant under changing ambient conditions, in particular changing temperature. The intended effect of the springs distributed over the circular circumference of the annular discs is to hold the discs together, and thus keep the filter gap width constant, with a force that is largely independent of ambient influences. Under certain operating conditions that can occur when the filters are actually being used in extraction operations, however, the springs behave in a way other than that desired. On account of the difference in pressure between the inflow side of the filter, which is generally on the outer circumferential surface of the annular discs, and the outflow side on the inner circumferential surface of the annular discs, axial compressive forces occur in the filter gap, it being possible for the axial forces to be considerable even when there is little difference in pressure, on account of the width of the annular discs. These axial forces may be greater than the resilient forces of the compression springs, which has the result that, as from a certain difference in pressure, the springs yield and one or more filter gaps change in an undesired way, which results in loss of the desired and intended filtering effect. It is not possible with the proposed structural designs to increase the spring pre-loading at will, since otherwise the Hertzian stress leads to rupturing of the ceramic filter rings even when the filter is not subjected to any loading.
With the compression springs, a spring force that is uniform over the circumference of the annular discs is exerted on the annular discs, offering a force of equilibrium to counter a very homogeneous isostatic pressure field inside or outside the filter. Tests with such filters show that, under technically realistic conditions, the compressive force fields are not homogeneous and the springs cannot prevent undesired tilting of the annular discs.
The compression springs can lose their intended effect to the extent that they lead to functional incapacity, or at least to failure of the intended filtering effect.
In the case of DE 10 2008 057 894 A1, WO2011/009469 A1 and WO2011/120539 A1, the annular discs are stacked in such a way that the spacers in the form of spherical segments must respectively lie one over the other. This technical solution has disadvantages to the extent that on the one hand assembly is complex, since the exact orientation of the rings has to be ensured, and on the other hand there is the risk of the filter becoming functionally incapable, because the rings twist under the effect of influences occurring when they are being transported or during operation.
In the case of WO2011/009469 A1, the brittle-hard annular discs have on their circumferential surface grooves for receiving guide rods, which serve for aligning and guiding the annular elements during assembly. In WO2011/120539 A1, the brittle-hard discs of the annular stack are held together by clamping rods that lie within the annular stack and are parallel to the longitudinal axis or a clamping tube that lies within the annular stack. On the inner circumferential surface, the brittle-hard discs have clearances or grooves for receiving the clamping rods. The grooves in the brittle-hard discs that are necessary for guiding the axially parallel clamping elements, like the grooves from WO2011/009469 A1, represent a significant mechanical weakening of the brittle-hard discs, since stress peaks occur at the grooves when the brittle-hard discs are subjected to the loading of external or internal pressures that occur during testing or operation. This leads to a lower load-bearing capacity of the filter system in terms of internal and external pressure.
It is known that temperatures of around 5000° C. prevail at the centre of the Earth. In the direction of the Earth's surface, there is a temperature gradient, with the result that boreholes generally become warmer at increasing depth. It is known from deep wells that, at a depth of 8000 meters, temperatures of about 250° C. can prevail. In extraction wells for oil and gas or else water, high temperatures must therefore be expected. The main need for separating devices that are used in extraction wells for oil and gas or else water is in the temperature range of up to 200° C. Separating devices that are used in extraction wells for oil and gas or water must therefore be capable of functioning in the temperature range of 10 to 200° C. When they are being transported and being stored, the separating devices may also be exposed to lower temperatures of down to −30° C., which the separating devices must be able to withstand undamaged.
It is therefore desirable to provide a wear-resistant separating device for the removal of solid particles from liquids, in particular from oil, gas and water, from extraction wells that has a great resistance to differences in pressure between the inflow side and the outflow side of the separating device. It is also desirable that the separating device withstands differences in temperature of at least 190° C., i.e. in the range of +10° C. to +200° C., during operation undamaged and without restricting its functional capacity. Furthermore, the separating device should be able to withstand undamaged the low temperatures occurring during transport and storage of down to −30° C. Furthermore, it is desired that the separating device can be used in curved extraction wells, is mechanically robust and meets the stringent requirements with regard to safety and reliability of the oil and gas industry.